In the production of rubbery materials by polymerization of one or more polymerizable monomers in a non-aqueous solvent with organometal initiators, the resulting product is obtained as a solution or suspension of rubber in the non-aqueous solvent, usually a rubber-in-hydrocarbon solvent solution termed a cement. Subsequently, the polymeric material must be recovered (separated) from the non-aqueous solvent. For this purpose, treatment of the cement with water has been convenient, usually by steam-stripping which has been most widely used on a commercial basis. Steam-stripping, usually assisted by one or more crumb-forming additives, removes the nonaqueous solvent or solvents and leaves a polymeric product in the form of a crumb as a slurry in water.
A similar net effect is involved in emulsion polymerizations, in that the product is a finely divided slurry of polymeric product in water.
Much of the water can be removed by filtration, pressing, or the like.
The product at this stage contains considerable percentages of water. If derived from a non-aqueous solvent polymerization system, a few percent of volatile and off times potentially flammable solvent still may remain associated with the polymeric/rubbery particles.
Various methods have been utilized in efforts to remove the final few percent of water, and, where present, the final few percent of remaining non-aqueous solvent.
One of the most common commercial methods employed is the so-called expansion process. In this process, the moist material is compressed in an extruder, such as a worm extruder which heats the material by work and friction, optionally with a supplementary supply of heat if desired, and is extruded through nozzles of relatively small diameter. The reduction in pressure which occurs on the discharge side of the die plate assists in evaporation of the moisture/solvent present.
Unfortunately, the normal collection method on the discharge side of the die plate has been to discharge the particles or strands into a relatively large receiving chamber or "hot box" which usually has a conveyor to remove the dry material. While various configurations have been tried, none have been particularly effective. The reason for this is that as the rubber or polymeric particles, crumbs, or strands, are sprayed out, impinging on the top and sides of the hot box, a buildup of polymer occurs on the surfaces of the chamber or hot box, usually on the upper portions of the sides and on the top of the hot box or expansion chamber. Being cooler, these surfaces, tend also to accumulate various proportions of moisture. The build-up of material lags in the chamber and tends to "age", resulting in degradative effects, or formation of gel from age, heat, and moisture. Periodically, various chunks of such materials fall back into the box and become associated with the fresh rubber or polymeric particles. These chunks of material, more or less moist, and of differing physical properties from the fresh product, may fall down onto the conveyor in the bottom of the "hot box", and ultimately be included in the pressed bales of products. As much as 5 to 10 percent by weight of the bale may be such undesirable product. This frequently requires either physical separation, or product rejection, resulting in increased costs to produce products without said undesirables in it.
Further, the accumulation on the surfaces of the "hot box" require periodic shutdown of the entire production train, and frequent manual cleaning. This in itself not only is a chore, but results in "track-in" of undesirable materials, grit, sand, and the like, from the outside as a consequence of the cleaning operation, and thus further contamination of the ultimate baled product.
Another problem has been with spontaneous fires due to flaming of the hydrocarbon solvent traces, resulting in overheating and fire hazards.
In spite of many difficulties, expansion drying has had advantages over other methods of drying, primarily being low cost of plant and operating costs. Nevertheless, distinct improvements are and have been needed.